Fix W3C Test Import script reformatting test HTML

Completely rewrite the test conversion process to minimize reformatting
when adding prefixes, etc. This isn't 100% perfect, there are still
places where it will end up changing the formatting, but it is much
better than before. Most notably, the public interface to the test
converter has changed: now one calls a method instead of creating an
instance of the test converter class. This is because the test converter
class now has state, so one really needs a new instance for each test.

Note that this also lays some simple groundwork for being able to use a
MockHost in the tests.

Review URL: https://chromiumcodereview.appspot.com/23995022

git-svn-id: svn://svn.chromium.org/blink/trunk@158036 bbb929c8-8fbe-4397-9dbb-9b2b20218538